The CoolMolecules database is ideal for helping students learn how to predict molecular shape from molecular or structural formulas. We use "sigma structures" (as shown on the right) for this purpose, but you could use Lewis structures. Sigma structures are simply the structures on the road to Lewis structures that have all electrons shown but no multiple bonds yet. See the Construct a Lewis Structure web page for detailed, step-by-step information about writing sigma structures.

The activity described below involves the discovery-mode learning we call data-driven chemistry. (See R. M. Hanson, S. A. Bergman, "Data-Driven Chemistry: Making Molecular Models (Literally) from Electron Diffraction Data," J. Chem. Educ. 1994, 150) Using this method, students start with sorting the data into different categories. Only after they start to see patterns in the data do they learn about the theory that explains those patterns. In this case, particularly using sigma structures, students quickly see that there is a correlation between the arrangement of bonds and lone pairs around the central atom and the geometry around the central atom.

Suggested Activity

A starting point for molecular shape prediction is to have students explore the database using the "shapes" search, making sigma structures for the molecules they find. For instance, a group of students could look up the trigonal planar molecules in the database and draw sigma structures for these molecules. It shouldn't be long before they come up with the idea, for example, that all trigonal planar molecules have three sigma bonds to the central atom, filled octets around the outer atoms, and no lone pair on the central atom.